Has auto AC been improved so much it’s no longer an energy sink?

October 1, 2010

Dear Cecil:

I recently bought my first new car in nearly 11 years. I had always bought non-air conditioned cars in the past because I was told AC reduces power and gasoline mileage. But now AC is standard equipment, at least in the make/model I bought, and the salesman assured me it no longer is an engine or mileage drain. Much as I enjoy the blasts of cool in the desert, this seems too good to be true. Is it?

— Tony Davis, Tucson

Cecil replies:

Of course. Do you think you can burn gasoline inside a metal box during an Arizona summer and get the thing cooled off for free? In fairness, however, there are subtleties that make the answer to this query more elusive than you’d think. Not to worry. For the easy questions you’ve got Google. For the tough ones you’ve got us.

The major changes in auto AC technology in recent years have been better compressors and refrigerants that are more environmentally friendly. The former are more efficient, the latter arguably less so; the net change probably isn’t dramatic. Of possibly greater significance is the fact that car body designs have become more aerodynamic.

Other researchers have had the same thought. Tony sent us a newspaper story from this past July saying GM wind-tunnel and proving-ground tests showed drag was negligible at city speeds, meaning you were better off with the AC off and the windows down. At speeds above 55 miles per hour, though, the opposite was true — open windows reduced gas mileage 20 percent while AC reduced it just 10 percent. The crossover point was 40 mph.

Here was a contention worth checking. Lab results were one thing; we needed to know how things worked in the real world. I sounded the klaxon in the Straight Dope Labs ready room and rousted out some volunteers. My assistant Dex had a Camry hybrid that measured gas mileage in tenths of a gallon. Fierra and Una, meanwhile, said they’d drive around in Fierra’s 2008 Corvette, which also calculated gas mileage in tenths, no doubt for the benefit of the many economy-minded individuals who buy muscle cars.

Each car was tested at 25, 40, and 55 mph, with multiple runs at each speed. The weather throughout was in the 80s and 90s. Here are the results, listed in this order: (1) AC off, windows up; (2) AC off, windows down; (3) AC on, windows up; (4) AC on, windows down. (Number-haters, visit http://chicago.straightdope.com/Mileage_test.JPG to see some easy-to-understand graphs.)

1. Running the AC meant a big mileage hit at virtually any speed. The one sort-of exception was the Corvette at 55 mph — with the AC on, fuel efficiency fell just 3 percent.

2. To our surprise, in some cases rolling down the windows without touching the AC made fuel efficiency go up. The most striking instance of this was the Corvette at 55 mph. With the AC off and windows up, it recorded 33.7 miles per gallon. When the windows were rolled down, gas mileage rose sharply to 37.2 miles per gallon. A fluke? Fierra and Una ran the test three times, always with the same result. With the AC on, the reverse happened: when the windows were up, the ’Vette managed 32.8 miles per gallon; when they were down it got only 29.6.

3. Dex experienced the same thing in his Camry at a lower speed. At 25 mph, his car displayed much better fuel efficiency with the windows down.

How do I explain this? For the moment, at least, I don’t. All I can say right now is what we might have predicted, Tony: unless you bought a Corvette, that car salesman’s AC spiel was hot air.

— Cecil Adams

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References

Calm, James "Emissions and environmental impacts from air-conditioning and refrigeration systems" International Journal of Refrigeration 25 (2002): 293–305.

Dane, M.H. et al. Investigation of Control Strategies for Reducing Mobile Air Conditioning Power Consumption University of Illinois at Urbana-Champaign Air Conditioning and Refrigeration Center, ACRC TR-199, August, 2002.